The present invention relates generally to apparatus and processes for effecting mass transfer between a liquid and a gas, and more particularly to a novel method and apparatus for effecting mass transfer between a liquid and a gas at substantially higher flow rates than have heretofore been obtainable.
Apparatus and techniques for effecting mass transfer in processes such as absorbtion, desorbtion, distillation and stripping are generally known. Many of the known apparatus are of the falling-film type in which a liquid flowing down the inside surface of a generally vertical cylindrical tube in a thin film absorbs a gas which is caused to flow internally of the tube in contacting relation with the liquid film. See, for example, U.S. Pat. No. 3,318,588.
It is recognized that mass transfer in processes such as absorbtion, desorbtion, distillation and stripping may be enhanced by increasing the velocity and hence the Reynolds number of gas flowing countercurrent to the falling liquid film with which the mass exchange is taking place. This is particularly pertinent where the exchange process between the liquid and gas is limited by the resistance on the gas side of the interface as the exchange within the gas increases with the Reynolds number of the gas flow. Further, since the rate of mass transfer is temperature dependent and may be either exothermic or endothermic, the ability to transfer heat simultaneously with the mass transfer to achieve temperature control is highly desirable.
Attempts have been made to increase the velocity of the flowing gas relative to the liquid film in the known mass transfer apparatus of the falling-film
For example, in the aforementioned U.S. Pat. No. 3,318,588, tubular gas velocity increasers are concentrically arranged within the upper ends of heat exchange tubes in a manner to form an annulus for the passage of the falling liquid film and flowing gas so as to increase the gas mass velocity. A limitation in mass transfer processes which employ the falling-film principle is the stability of the falling liquid film in the presence of the counterflowing gas. As the gas flow increases for a particular liquid flow rate, waves appear on the liquid surface and the liquid film becomes more unstable with further increase in gas velocity so as to result in the phenomenon of flooding. At the flooding condition, the flow becomes chaotic, the liquid film breaks up and the liquid becomes entrained in the gas flow. In the case of gas flowing upwardly within the falling-film tubes, some of the liquid entrained in the gas flow is expelled from the top of the tube.
It therefore follows that delaying the onset of flooding will enable higher gas velocities and, accordingly, a greater turbulent exchange within the gas and at the liquid-gas interface. One attempt at obtaining more intensive transfer between contacting phases is disclosed in U.S. Pat. No. 4,009,751 wherein apparatus in the form of a smooth cylindrical tube has a helical insert on the internal wall of the tube to guide the liquid phase flow in a helical path down the internal wall surface of the tube, and has a second helical insert in the form of a twisted tape disposed generally axially of the tube to establish a rotational flow of the lighter phase gas in a rotational direction opposite to the rotational flow imparted to the liquid phase. Spiral flow of the liquid phase in a falling-film type heat exchanger is also taught by U.S. Pat. No. 2,545,028 in which a falling liquid film is introduced through an annulus at the upper end of a cylindrical flow passage while undergoing substantially tangential flow.